Introduction: A rapid and sensitive COVID-19 diagnostic test is required to aid in the prevention and control of the current COVID-19 pandemic spread. We developed a colorimetric, rapid, and sensitive RT-LAMP assay for the diagnosis of COVID-19 viral infection. Methodology: Complete genome sequences of 41 SARS-CoV-2 isolates from Oman were used in this study. Three primer sets (CoV_S1, CoV_S2, CoV_M1) were developed from all Omani SARS-CoV-2 genome sequences available at the time, targeting the spike protein gene and the M gene. The primer set (CoV_S1) was found to be the most sensitive and specific among the three designed sets. The sensitivity and specificity of the assay were compared to that of qRT-PCR. Direct testing of SARS-CoV-2 spiked saliva with the developed assay was evaluated. Lyophilized colorimetric assays were stored at room temperature and 4 °C and their ability to detect positive samples were tested for a period of 8 weeks. Results: The RT-LAMP assay was validated by testing 145 COVID-19 clinical samples with a sensitivity of 96.9% and specificity of 94.7% when compared to the validated qRT-PCR assay. The assay specificity was tested against SARS-CoV Frankfurt 1 RNA virus and avian coronaviruses as they tested negative with the developed assay. The assay was lyophilized and managed to detect the positive samples colorimetrically when stored at 4 ̊C for up to 8 weeks. Conclusions: The assay can be utilized in its current form as a screening assay with the advantages of being simpler, quicker, and cheaper than the qRT-PCR.